Merging multiple PDF files into one

Try

gs -q -sPAPERSIZE=a4 -dNOPAUSE -dBATCH \
   -sDEVICE=pdfwrite -sOutputFile=out.pdf in1.pdf in2.pdf in3.pdf

or, perhaps even better, install pdftk and use

pdftk in1.pdf in2.pdf in3.pdf cat output out.pdf